Tool holder for a tool that can rotate about an axis of rotation

ABSTRACT

A tool holder is disclosed for a boring, milling or reaming tool that can rotate about an axis of rotation. At the end of its clamping shank that is located on the side of the tool, the tool holder has a bush part with a mounting opening, which is centered with regard to the axis of rotation and which is provided for accommodating a fixing shank of the tool. The mounting opening for a bush part can be radially expanded to a width, which enables the fixing shank to be inserted or removed, particularly by means of eddy currents that are magnetically induced inside the bush part by an induction coil. In addition, the mounting opening can, by cooling, be radially contracted to a width that holds the fixing shank with an interference fit.

The invention relates to a tool holder for a tool that can rotate aboutan axis of rotation, in particular a drilling, milling or reaming tool.

It is known, for example from U.S. Pat. No. 5,311,654, to hold thefixing shank of a drilling, milling or reaming tool in an interferencefit in a mounting opening which is concentric to its axis of rotation ofa bush part which is closed in an annular manner. The bush part formsthe tool-side end of a conventional tool-holder for connection to therotating spindle of the machine tool and, by heating, for example bymeans of eddy currents magnetically induced in the bush part, can beradially expanded to a width enabling the fixing shank of the tool to bepushed in or pulled out. When the fixing shank is pulled out, the insidediameter of the mounting opening of the tool holder, at the workingtemperature of the machine tool, is slightly smaller than the outsidediameter of the fixing shank of the tool, so that, after the cooling ofthe bush part, expanded by heating, to the working temperature of themachine tool, the fixing shank is shrunk in place in the bush part. Thefrictional forces transmitted in the shrink fit are sufficiently largein order to be able to transmit, despite the absence of positivelocking, the torque required for the machining operation from the toolholder to the tool.

During the insertion of the fixing shank into the tool holder, the bushpart can be sufficiently heated before the fixing shank is exposed tothe heat source. However, if the fixing shank is to be pulled out of thebush part again, the bush part must be heated so quickly that the regionof the mounting opening expands more quickly than the fixing shank ofthe tool, the fixing shank in this case likewise being exposed to theheat source. To pull the fixing shank out of conventional tool holders,comparatively high tensile forces are therefore often required.

The object of the invention is to improve the design of a tool holder,holding the fixing shank of a rotating tool in a shrink fit, in a simplemanner in such a way that the tool can be removed again from the toolholder without any problems.

From a first aspect, the invention is based on a tool holder for a toolthat can rotate about an axis of rotation, in particular a drilling,milling or reaming tool, said tool holder comprising:

-   a clamping shank which, at its tool-side end, has a bush part closed    in an annular manner and having a mounting opening, concentric to    the axis of rotation, for a fixing shank of the tool, the mounting    opening, by heating the bush part, in particular by means of eddy    currents magnetically induced in the bush part, being radially    expandable in a way enabling the fixing shank of the tool to be    pushed in or pulled out and being radially shrinkable, by cooling,    to a width holding the fixing shank in an interference fit.

The above object is achieved according to the first aspect of theinvention in that the bush part has heat-insulating means which, duringheating of a radially outer circumferential region of the bush part, inparticular by means of eddy currents magnetically induced in the bushpart, reduce the thermal conductivity of a radially innercircumferential region of the bush part compared with the radially outercircumferential region, or/and in that cooling means are assigned to thebush part, these cooling means, during heating of a radially outercircumferential region of the bush part, in particular by means of eddycurrents magnetically induced in the bush part, reducing the rate ofchange of the temperature increase in a radially inner circumferentialregion of the bush part.

The heat-insulating means or/and the cooling means ensure that theradially inner circumferential region, that is to say that region of thebush part which is next to the clamping surface of the mounting opening,is heated less quickly than the radially outer circumferential region,which is closer to the external heat source, of the bush part. In thisway, the heating of the radially inner circumferential region and thusof the fixing shank of the tool is delayed. Since the radially innercircumferential region and the radially outer circumferential region arefirmly connected to one another, the radial tensile forces producedduring the expansion of the radially outer circumferential region alsoexpand the radially inner circumferential region at the same time, sothat the fixing shank of the tool is released from its interference fitvery quickly and without being excessively heated itself.

The bush part is preferably heated by eddy currents, magneticallyinduced in the bush part, by means of an induction coil which enclosessaid bush part and carries an alternating current. The bush part is madeof metal, that is to say of an electrically conductive material, and ispreferably also magnetically conductive in order to concentrate themagnetic flux of the induction coil upon itself. In a preferredconfiguration, a plurality of apertures reducing the radialheat-conducting cross section of the bush part are provided in theradially inner circumferential region of the bush part. These aperturesreduce not only the heat-conducting cross section but also the magneticcircuit cross section available for the magnetic flux of the inductioncoil, with the result that the eddy currents heating the bush part areconcentrated on its radially outer circumferential region. Similareffects occur if the apertures are intended for heat-exchange contactwith a gaseous or liquid coolant.

The torque which can be frictionally transmitted from the bush part tothe fixing shank of the tool depends on the lateral surface area, whichcan be brought into contact with the fixing shank, of the mountingopening. In order to ensure as large a lateral surface area of themounting opening as possible, the apertures are preferably designed aspassages running entirely in the material of the bush part. In thiscase, the radially inner circumferential region, forming the mountingopening for the fixing shank of the tool, of the bush part is firmlyconnected, in particular in one piece and homogeneously, to the radiallyouter circumferential region by material webs situated between thepassages. The passages may run essentially parallel to the axis ofrotation, and several rows of passages may be provided in a staggeredmanner in the radial direction in order to increase the total passagecross section.

For the thermal insulation, the apertures or passages may contain air;however, they may also be filled with a thermally insulating solidmaterial.

To cool the radially inner circumferential region of the bush part, theapertures or passages may be connected to a coolant delivery device,such as, for example, a cooling-air fan or a cooling-water source, sothat the bush part, during the heating, can at the same time be cooledin a positive manner in its radially inner region. Given an adequatecooling capacity, the thermally insulating effect of the passages maypossibly also be negligible.

In the preferred configurations explained above, the radially innercircumferential region of the bush part is firmly connected to theradially outer bush part, so that the radially outer bush part, duringthe thermal expansion, can exert radial tensile forces on the radiallyinner bush part. In a preferred configuration, however, theheat-insulating means may already form the radially innercircumferential region. It is also not necessary to transmit radialtensile forces in every case, provided provision is made to ensure thatthe radially inner bush part forming the heat-insulating means cancertainly transmit compressive forces for the shrink-fitting process butif need be can elastically expand during the expansion for theunshrinking of the fixing shank of the tool. Suitable embodiments, suchas, for example, corrugated bushes or bushes slotted in the axialdirection, are those which can be pressed by the radially outercircumferential region of the bush part for the shrink fit against thefixing shank of the tool, but which at most surround the fixing shankwith their radial elasticity during the expansion of the radially outercircumferential region and thus release the fixing shank during theexpansion of the outer circumferential region. Such a bush body formingthe heat-insulating means may be made, for example, of ceramic.

The idea explained above of designing the heat-insulating means as abush body which can be separated from the tool holder, irrespective ofheat-insulating and/or cooling properties, also has advantages both forthe production cost of the tool holder and for the problem-freeunshrinking of the tool from the tool holder. From a second aspectindependent of the first-mentioned aspect, the object of the inventionis to improve the design of a tool holder, holding the fixing shank ofthe rotating tool in a shrink fit, in a simple manner in such a way thatthe tool can be removed again from the tool holder without any problems.

From the second aspect, too, the invention is based on a tool holder fora tool that can rotate about an axis of rotation, in particular adrilling, milling or reaming tool, said tool holder comprising: aclamping shank which, at its tool-side end, has a bush part closed in anannular manner and having a mounting opening, concentric to the axis ofrotation, for a fixing shank of the tool, the mounting opening, byheating the bush part, in particular by means of eddy currentsmagnetically induced in the bush part, being radially expandable to awidth enabling the fixing shank of the tool to be pushed in or pulledout and being radially shrinkable, by cooling, to a width holding thefixing shank in an interference fit.

The second aspect of the invention is characterized in that a bush bodytransmitting the radial interference-fit forces of the bush part to thefixing shank is inserted into the mounting opening, the bush wall ofthis bush body being closed in an annular manner in the circumferentialdirection, at least in the wall region transmitting the interference-fitforces, and having a multiplicity of axially elongated passages arrangednext to one another in the circumferential direction.

Irrespective of whether the bush body has heat-insulating properties orcooling means are assigned to it, the production of the tool holder isconsiderably simplified, since the bush part, forming the mountingopening, of the tool holder runs on a larger diameter than the bush bodyinserted into the mounting opening. The bush part of the tool holdertherefore expands to a greater degree during the heating and permitsradially elastic yielding of the bush body, even if the latter has stillnot achieved a thermal expansion releasing the fixing shank of the tool.On account of the improved expansion behavior, the bush body can beproduced with comparatively large tolerances and can thus be produced ina simple manner without impairing the unshrinking behavior, inparticular from tool shanks having a small diameter. Since the bush bodyaccording to the invention has a bush wall closed in an annular mannerin the circumferential direction, high torques can also be transmittedwith high precision from the tool holder to the shank of the tool.Unlike in the case of bush bodies which are slotted in a radiallycontinuous manner, the torque which can be transmitted is not reduced oris only reduced slightly.

The bush body can be produced in a comparatively simple manner if thepassages are designed as axially elongated recesses which are arrangedon the outer lateral surface or/and on the inner lateral surface of thebush wall and are open toward the outer lateral surface or toward theinner lateral surface but are closed to the respectively other lateralsurface. The recesses are expediently designed as radial slots havingslot walls essentially parallel to one another and are preferablynarrower in the circumferential direction of the bush body than in theradial direction in order to weaken the bush cross section transmittingthe torque as little as possible.

The recesses, in the manner of a corrugated bush, may be providedalternately on the outer lateral surface and the inner lateral surface,in which case, if need be, recesses adjacent in the circumferentialdirection may overlap one another radially.

Alternatively, however, the recesses may also be arranged only on theinner lateral surface of the bush lateral surface, or else the passagesmay be situated entirely in the bush wall and be designed, for example,as axial bores. If the slots are provided only on the inner lateralsurface of the bush lateral surface, the wall region closed in anannular manner remains at the outer circumference, a factor whichimproves the expansion behavior of the bush body.

The passages preferably open out freely at the surface of the bush bodyaxially on both sides of the wall region transmitting the pressingforces, in particular at the end faces of said bush body or else atlateral surface regions in which they are no longer covered by thefixing shank of the tool or by the mounting opening of the bush part. Inthis way, the passages, during operation of the tool, can at the sametime be used in a manner known per se for feeding lubricant or coolantto the tool.

The mounting opening of the bush part preferably has acircular-cylindrical inner surface extending essentially completely upto a tool-side end face of the bush part. It goes without saying thatthe mounting opening, in this connection, may also be regarded asextending completely up to the end face when the edge of the end facemerges into the mounting opening with a slight chamfer of, for example,less than 1 mm axial length. In the case of a bush part of such adesign, provision is preferably made for the bush body to be completelyinserted axially into the mounting opening, so that its wall regiontransmitting the pressing forces extends essentially completely up tothe tool-side end face of the bush part. Here, too, slight chamfers maybe provided. An advantage of such a configuration is that the bush body,on the tool side, can be used essentially completely for transmittinginterference-fit forces.

The bush body is preferably axially longer than its wall regiontransmitting the interference-fit forces. On its side axially remotefrom the tool, the bush body in each case has an inner lateral surfaceregion enclosing the fixing shank of the tool at a radial distance. Thishas the advantage that the bush body can be better fixed in the mountingopening of the bush part of the tool holder axially outside its wallregion transmitting the interference-fit forces, and in addition thislengthening of the bush body can be used for guiding the tool shankduring the shrink fitting and tilting can be reliably avoided. It goeswithout saying that the radial distance in this case may also be verysmall.

The axial length of that wall region of the bush body which transmitsthe interference-fit forces is expediently selected in accordance withthe diameter of the tool shank. A suitable axial length is preferablybetween 2 to 5 times, in particular about 3 times, the diameter.

Since the bush body need not have thermal insulating properties, it ispreferably made of high-temperature steel.

The bush body explained above increases the effective diameter, for theshrink-fit expansion, of the bush part of the tool holder for a givennominal diameter of the tool shank and thus reduces the requirements formaintaining production tolerances, in particular in the case of smalldiameters. Corresponding advantages can also be achieved from a thirdaspect of the invention if the slots increasing the effective shrink-fitdiameter relative to the nominal size are incorporated directly in thebush part, which is otherwise closed in an annular manner; the bush bodyis thus connected to the bush part virtually integrally and in onepiece.

From the third aspect, the tool holder for a tool that can rotate aboutan axis of rotation, in particular a drilling, milling or reaming tool,again comprises a clamping shank which, at its tool-side end, has a bushpart closed in an annular manner and having a mounting opening,concentric to the axis of rotation, for a fixing shank of the tool, themounting opening, by heating the bush part, in particular by means ofeddy currents magnetically induced in the bush part, being radiallyexpandable to a width enabling the fixing shank of the tool to be pushedin or pulled out and being radially shrinkable, by cooling, to a widthholding the fixing shank in an interference fit, and is characterized inthat the bush part has a multiplicity of axially elongated radial slotsarranged next to one another in the circumferential direction and opentoward the mounting opening and is closed in an annular manner in itscircumferential region radially adjoining the slots on the outside andmerges in one piece into web regions remaining between the slots.

Also from this aspect of the invention, it is not important for theradially inner circumferential region of the bush part to be thermallyinsulated from the radially outer circumferential region. However, thethermal insulation may be present. The slots shift the diameter of thebush part effective for the thermal expansion radially outward andincrease its effective diameter relative to the nominal diameter of themounting opening by their radial depth. The slots preferably begin atthe tool-side end of the bush part and extend in the axial directionover at least 5 to 10 mm, since the unshrinking problems are greatesthere, but preferably over at least the length of that region of themounting opening which transmits the interference-fit forces and may ifneed be have a radial depth changing in the axial direction, inparticular a radial depth decreasing from the insertion side of the toolshank toward the clamping shank. The radial depth of slots adjacent inthe circumferential direction may also be different.

In order to reduce the contact area, transmitting the torque, of themounting opening as little as possible, the width of the slots in thecircumferential direction is as small as possible. The slots expedientlyhave a width in the circumferential direction of between 0.1 mm and 0.5mm.

The radial depth of the slots determines the increase in that diameterof the bush part which is effective for the thermal expansion. Theradial depth is measured in such a way that an effective expansiondiameter is obtained, which allows the mounting opening to be producedwith sufficiently large tolerances, which are thus simple to realize.Since the effective expansion diameter of the bush part is to be all thegreater, the smaller the nominal diameter of the mounting opening is,provision is made in a preferred configuration for the radial depth ofthe slots in that region of the mounting opening which transmitsinterference-fit forces, at a nominal diameter of the mounting openingless than or equal to 10 mm, to be greater than 0.1 times the nominaldiameter, preferably equal to or greater than 0.2 times the nominaldiameter. At a nominal diameter of the mounting opening less than orequal to 6 mm, the radial depth of the slots is preferably greater than0.15 times the nominal diameter, but better equal to or greater than 0.3times the nominal diameter, and, at a nominal diameter of the mountingopening less than or equal to 3 mm, the radial depth is expedientlygreater than 0.2 times the nominal diameter, but preferably equal to orgreater than 0.5 times the nominal diameter. These dimensions ensurethat the web regions of the bush part which remain between the slots aresufficiently flexurally rigid during torque loading.

The number of slots arranged in the circumferential direction preferablyat equal angular distances is to be a compromise between sufficientbearing capacity of the web regions remaining between the slots on theone hand and the improvement in the expansion behavior of the bush parton the other hand. At least six slots, but better at least eight slots,are expediently provided. However, there are expediently fewer than 20slots.

Preferred exemplary embodiments of the invention are explained in moredetail below with reference to a drawing, in which:

FIG. 1 shows an axial longitudinal section through a tool holderaccording to the invention;

FIG. 2 shows an axial cross section through the tool holder along lineII—II in FIG. 1;

FIGS. 3 to 8 show axial cross sections through the bush part of variantsof the tool holder from FIG. 1;

FIG. 9 shows a perspective representation of a bush body which can beinserted in the tool holder for heat insulation;

FIG. 10 shows an axial longitudinal section through the bush part,intended for accommodating the tool shank, of a variant of a tool holderaccording to the invention;

FIG. 11 shows an axial longitudinal section through a bush body insertedinto the tool holder according to FIG. 10;

FIG. 12 shows an axial cross section through the bush body along lineXII—XII in FIG. 11;

FIG. 13 shows an axial longitudinal section through a variant of thebush body from FIG. 11;

FIG. 14 shows an axial cross section through the bush body along lineXIV—XIV in FIG. 13;

FIG. 15 shows an axial cross section through a further variant of thebush body;

FIG. 16 shows an axial longitudinal section through the bush part,intended for accommodating the tool shank, of a further variant of atool holder according to the invention, and

FIG. 17 shows an axial cross section through the bush part along lineXVII—XVII in FIG. 16.

The tool holder shown in FIG. 1 has a clamping shank 1 which in thiscase is in one piece but if need be is also composed of a plurality ofpieces and which has at one of its ends a conventional spindle coupling3, here in the form of a standard taper, with which it can be insertedin a rotationally locked manner and coaxially to its axis of rotation 5into a corresponding receptacle of a rotating work spindle of a machinetool. At its end axially remote from the work spindle, the clampingshank 1 has a bush part 7 which is concentric to the axis of rotation 5,is closed in an annular manner, is made in a homogeneous manner of aferromagnetic material, for example steel, and contains a mountingopening 9 which is concentric to the axis of rotation 5 and isessentially cylindrical in this case and which is intended for a fixingshank, indicated at 11, of a rotating tool, for example a drill, millingcutter or a reaming tool. The nominal inside diameter of the cylindricalmounting opening 9 is slightly smaller than the nominal outside diameterof the fixing shank 11, so that the bush part 7 is able to hold thefixing shank 11, inserted into the mounting opening 9, in atorque-transmitting manner in an interference fit or shrink fit withoutadditional positive-locking elements.

The bush part 7, which is cylindrical at its outer circumference in theexemplary embodiment shown, but if need be also tapers conically towardthe tool, can be heated inductively by means of an induction coil whichencloses the bush part 7 on the outside, is indicated at 13 and carriesan alternating current. The induction coil extends over most of theaxial length of the bush part and induces eddy currents in theelectrically and magnetically conductive material of the bush part 7,and these eddy currents, in addition to the magnetic losses of themagnetic hysteresis of the material of the bush part, electrically heatthe bush part. Due to the heating, the bush part and thus the mountingopening expand radially, with the result that the fixing shank 11, inthe heated state of the bush part 7, can be inserted into or pulled outof the mounting opening 9. If the bush part 7 is cooled down to theworking temperature of the machine tool, for example to ambienttemperature, when the fixing shank 11 is inserted, the bush part 7 isshrunk onto the fixing shank 11 in an interference fit.

The design of the induction coil 13 and of the alternating-currentgenerator required for its operation is known per se and is not to beexplained further here. Since the induction effect of the induction coil13 increases with increasing frequency, a higher-frequency alternatingcurrent is preferred for feeding the induction coil.

The fixing shank 11 of the tool, in a similar manner to the bush part 7,is usually made of both an electrically and magnetically conductivemetal, such as steel for example, and would likewise be heated byinduced eddy currents. Whereas the bush part 7, during the insertion ofthe fixing shank 11, can be inductively heated before the fixing shank11 is brought sufficiently close to the magnetic field, it must beensured when removing the tool that the bush part 7 expands more quicklythan the fixing shank 11, which is likewise exposed to the heating. Sothat the heating of a radially inner circumferential region 15 of thebush part 7 which is adjacent to the mounting opening 9 is delayedrelative to the heating of a radially outer circumferential region 17 ofthe bush part 7, the bush part 7, close to the mounting opening 9,contains a multiplicity of passages 19 which run parallel to the axis ofrotation 5 and radially reduce the heat-conducting cross section of thebush part 7 and thus thermally insulate the radially innercircumferential region 15 from the radially outer circumferential region17. The passages 19 contain air, but may also be filled with anotherthermally insulating material, such as, for example, solid insulatingmaterial, for instance heat-resistant plastic. Remaining between thepassages 19 are web-like material regions 21, which firmly connect theradially inner circumferential region 15 to the radially outercircumferential region 17 in an integral manner. If the circumferentialregion 17 is thermally expanded, the material webs 21 transmit radialtensile stresses to the inner circumferential region 15. On account ofthe delayed heating of the inner circumferential region 15, the outercircumferential region 17 can be heated to a greater extent thanhitherto, with the result that the bush part 7 can be expanded to agreater inside diameter than hitherto without the fixing shank 11 of thetool following with the same rate of change. In this case, too, theexpansion is effected more quickly than hitherto.

The passages 19 may be designed as chambers closed all round, but areexpediently open at least at one of their ends, here the tool-side end,in order to permit a cooling convection flow. Both ends of the passages19 are preferably open directly or indirectly to the environment of thetool holder and allow a coolant to flow through the bush part 7. In thetool holder of FIG. 1, the end of the passages 19 which is remote fromthe tool is connected individually, in groups or jointly for all thepassages to at least one further passage 23, which, as indicated by anarrow 25, permits a positive flow through the bush part 7, at leastduring the inductive heating phase of the tool unshrinking operation.Via the passage 23, cooling air can be blown through the passages 19,for example by a cooling-air fan. It goes without saying that a coolingliquid, for example cooling water, may also be used for cooling the bushpart 7, this cooling water, if need be, being pumped through thepassages 19 in a closed cooling-water circuit. FIG. 1 shows the coolantflow toward the tool side of the tool holder. The direction of flow mayof course be the other way round. The passages 19, instead of beingconnected to the outer circumference of the clamping shank 1, may alsobe connected to a central bore 26 of the clamping shank 1, which as arule is present anyway in the clamping shank 1 and extends right intothe mounting opening 9.

In the tool holder of FIGS. 1 and 2, the passages 19 are designed ascircular-cylindrical bores. Variants of axially normal cross-sectionalshapes of the passages are explained below, as may advantageously beprovided instead of the circular-cylindrical passages of FIGS. 1 and 2.Components having the same effect are designated with the referencenumerals of FIGS. 1 and 2 and are provided with a letter fordifferentiation. To explain the construction and the functioning,reference is made in each case to the entire description.

FIG. 3 shows a variant in which the passages 19 a extending parallel tothe axis of rotation 5 a have a circle segment shape in the axiallynormal cross section and are arranged concentrically to the axis ofrotation 5 a on two diametral circles having different diameters. Thepassages 19 a arranged on the two diametral circles are distributed insectors in the circumferential direction, so that radially extendingwebs or spokes 21 a remain between the radially outer circumferentialregion 17 a and the radially inner circumferential region 15 a, andthese webs or spokes 21 a connect these two circumferential regions ofthe bush part 7 a to one another for the transmission of tensile forces.It goes without saying that the radial orientation of the webs is alsoadvantageous in the variant of FIG. 2. Whereas in the embodiment in FIG.2 the radially inner circumferential region 15 is closed in thecircumferential direction, slots 27 running at least over part of theaxial length of the bush part 7 are provided in the variant in FIG. 3between webs 21 a which are adjacent in the circumferential direction,these slots 27 reducing the tensile forces which are required forexpanding the radially inner circumferential region 15 a. The slotsconnect the mounting opening 9 a to passages 19 a on the smallerdiametral circle. Here, and in the individual case below, the slots 27may be dispensed with. To stabilize the annular wall 29 separating thetwo rows of passages 19 a from one another, the passages 19 a on thelarger diametral circle are shorter in the circumferential directionthan the passages 19 a arranged on the circle of smaller diameter, sothat the wall 29 remains connected to the radially outer circumferentialregion 17 a via webs 31 which remain in between.

FIG. 4 shows a variant which differs from the embodiment in FIG. 3merely by the fact that only one circumferential row of passages 19 b isprovided, the method of arranging them corresponding to the passages ofthe circle in FIG. 3 arranged on a smaller diameter.

In the embodiments explained above, the bush part has a circular shapein axial cross section. FIG. 5 shows a variant in which the bush part 7c has a polygonal shape in axial cross section, here the shape of asquare with a boundary rounded in a crowned manner. Accordingly, thepassages 19 c also extend in an elongated manner along the boundary andhave a lenticular cross-sectional contour.

In the embodiments in FIGS. 3 to 5, the cross-sectional shape of thepassages is longer in the circumferential direction of the bush partthan in the radial direction. FIG. 6 shows a variant in which thepassages 19 d are designed as slot structures which are elongated in theradial direction and whose width in the circumferential direction issmaller than their length in the radial direction. As in the embodimentsexplained above, the slots 19 d extend in an axially parallel manner tothe axis of rotation 5 d, webs 21 d which remain between the slots 19 dagain integrally connecting the radially outer circumferential region 17d of the bush part 7 d to the radially inner circumferential region 15d. The radially inner circumferential region 15 d is closed in anannular manner in the circumferential direction.

In the embodiment in FIG. 6, the passages 19 d have an approximatelyuniform width over their radial height. The webs 21 d remaining betweenthe passages 19 d are accordingly wider radially on the outside thanradially on the inside, with the result that the heat-conducting crosssection decreases only gradually toward the radially innercircumferential region 15 d. FIG. 7 shows a variant in which thepassages 19 e are adjacently wider in the circumferential direction inthe circumferential region 17 e than on their side facing thecircumferential region 15 e. The webs 21 e accordingly have anessentially uniform circumferential width over their radial height and,for reducing their heat-conducting cross section, additionally have aslot 33 extending radially inward from the mounting opening 9 e betweenadjacent passages 19 e. The slot 33 extends in a radial plane parallelto the axis of rotation 5 e. In this way, the inner lateral surface ofthe mounting opening 9 e is divided into a plurality of segments.

FIG. 8 shows a variant which differs from the embodiment in FIG. 7 onlyby the axial cross-sectional shape of the passages 19 f. Whereas thepassages 19 e in FIG. 7 have longitudinal edges rounded in the shape ofan arc of a circle in axial cross section toward the circumferentialregions 17 e and 15 e, these longitudinal edges in the variant in FIG. 8run on diametral circles about the axis of rotation 5 f. The passages 19f accordingly have an approximately trapezoidal shape.

In the embodiments explained above, the radially outer circumferentialregion 17 of the bush part 7 is connected to the radially inner region15 via material webs so as to be resistant to tensile force. Such aconnection resistant to tensile force is unnecessary if the radiallyinner region 15, as indicated at 35 (FIG. 1), is designed as a bush bodywhich can transmit radial compressive forces of the outercircumferential region 17 to the fixing shank 11 and has heat-insulatingproperties. Properties which transmit the tensile force between theouter circumferential region 17 and the inner circumferential region 15may be dispensed with if this bush body 35 can be expanded at leastslightly elastically in the circumferential direction, so that, when theradially outer circumferential region 17 is thermally expanded forpulling out or inserting the fixing shank 11, at most the low retainingforces of the bush body 35 have to be overcome. The bush body 35 may bedesigned, for example, as a corrugated bush which is made of an elasticmaterial, such as steel for example, and whose corrugation at the sametime forms the heat-insulating/coolant passages explained above. Axiallyslotted bushes are also suitable, at least one axially continuous slotproviding for the elastic expandability of the bush, or if need be aplurality of axially discontinuous slots producing a radially elastictongue structure or a radially elastic meander structure of the bushbody. These bush bodies may also be made of metal. However, bush bodiesmade of ceramic material are also especially suitable. FIG. 9 shows abush body of the last-mentioned type having a single slot. The slot canbe seen at 37. In addition, axially discontinuous slots which formtongues 41 between them are shown at 39 in FIG. 9. Embodiments having anadditional bush body 35 also have the advantage that they permit greaterexpansions of the mounting opening 9 than hitherto, since the radiallyouter circumferential region 17 producing the interference fit runs on alarger outside diameter than would be the case with bush parts whichextend directly up to the fixing shank 11 of the tool.

FIGS. 10 to 12 show a variant of the previous tool holder 1 explainedwith reference to FIG. 1, in the bush part 7 of which a bush body 51similar to the bush body 35 is inserted. To accommodate the bush body51, the bush part 7 again has a circular-cylindrical mounting opening 53which is concentric to the axis of rotation 5 and is formed by an outercircumferential region 17, closed in an annular manner, of the bush part7. The bush body 51 on its own, i.e. when the tool is not yet clamped,sits in the mounting opening 53 in a radially elastic manner infrictional connection, to be precise in such a way that the bush body 51can be exchanged in operation. The radial elasticity of the bush body 51is so great that the bush body 51 can follow a thermal expansion of thecircumferential region 17 of the bush part 7 by means of an inductioncoil, as has been explained above with reference to FIG. 1.

The bush body 51, as best shown by FIGS. 11 and 12, is designed as acorrugated body over its entire axial length and has a bush wall 55closed in the circumferential direction. Alternating in thecircumferential direction, axially elongated slots 61 start from theouter lateral surface 57 and from the inner lateral surface 59 of thebush wall 55 and extend in radial longitudinal planes in each case withslot walls parallel to one another but do not extend up to the otherlateral surface 59 or 57, respectively. The depth of the slots 61 in theradial direction is greater than their width in the circumferentialdirection, slots which are adjacent in the circumferential directionoverlapping one another radially. In order to weaken the wall crosssection as little as possible, the slots 61 are between 0.1 and 0.3 mmwide, and viewed overall there are between 10 and 20 slots 61,specifically 16 slots 61 in the exemplary embodiment shown, whichresults in an adequate compromise between radial bearing capacity of thebush body 51, its torque-transmitting capacity and its radialelasticity. Since the bush wall 55 is closed in the circumferentialdirection on account of the meander or corrugated structure, the bushbody 51 can transmit a comparatively high torque from the bush part 7 tothe tool shank 11.

As FIG. 11 shows, the inner lateral surface 59, machined in acircular-cylindrical manner at least when the bush body 51 is insertedinto the mounting opening 53, is stepped in the direction of the axis ofrotation 5 and, in its region 63 facing the insertion side of the toolshank 11, has an inside diameter which allows interference-fit forces tobe transmitted from the bush part 7 to the tool shank 11. The region 63allowing the transmission of interference-fit forces to the tool shank11 has an axial length of between two to five times, here about threetimes, the diameter of the inner lateral surface 59. Directed axiallyaway from the insertion side of the tool shank 11, the bush body 51 islengthened in a region 65, in which the diameter of the inner lateralsurface 59′ is slightly enlarged with a step 67 being formed, so thatthe inner lateral surface 59′ runs at a slight radial distance from thetool shank 11 when the latter is held in an interference fit in theregion 63. The lengthening region 65 of the bush body 51 improves thetorque-transmitting capacity of the bush body 51 and provides for slightaxis tilting errors of the tool shank 11 during the shrink fitting ofthe latter.

As FIG. 10 shows, the bush body 51 is inserted essentially completelyinto the mounting opening 53 of the bush part 7 and terminatesapproximately flush with its end face 69. The region 63 transmittinginterference-fit forces is thus completely enclosed by the bush part 7,so that the bush body 51 can transmit pressing forces in the entireregion 63. In addition, the free length of the tool shank 11 whichprojects beyond the bush part 7 is reduced to the minimum. The region 65enlarged in diameter, as FIG. 10 also shows, accommodates an adjustingscrew 71 which can be screwed in the bush part 7 and serves as an axialstop for the defined shrink fitting of the tool shank 11. Insofar as theabove bush body 51 terminates flush with the end face 69, this alsoincludes a termination with chamfers, as are indicated, for example, at73.

The bush body 51 and the bush part 7 are expediently made of the samematerial, for example high-temperature steel. The slots 61 primarilyprovide for the radial elasticity of the bush wall 55, otherwise closedin an annular manner, without appreciably reducing the thermalconductivity of the bush body 51. The improvement in the unshrinkingbehavior of a tool holder provided with the bush body 51 thereforeresults primarily from the greater expansion of the region 17 of thebush part 7 as a result of the greater radius.

FIGS. 13 and 14 show a variant of the bush body 51 which can be used ina corresponding manner instead of this bush body in the tool holder 1 inFIG. 10. Components having the same effect are designated with thereference numerals of FIGS. 10 to 12, supplemented by a letter, here“g”. To explain the construction and the functioning, reference is madeto the description with regard to FIGS. 10 to 12 and additionally to thedescription with regard to FIG. 1.

Unlike the bush body 51 in FIGS. 10 to 12, only slots 61 g which startfrom the inner lateral surface 59 g are provided in the bush wall 55 gof the bush body 51 g in FIGS. 13 and 14. Here, too, the circumferentialwidth of the 10 to 20, here 16, slots 61 g, which may be between 0.1 and0.3 mm, is considerably smaller than the radial depth of the slots 61 g.The outer lateral surface 57 g of the bush body 51 g is closedcylindrically. Since the slots start from the inner lateral surface 59g, the outer lateral surface 57 g, which is extensible to a greaterdegree on account of its greater circumferential length, is availablefor the radial expansion. Here, too, the region of the inner lateralsurface is stepped in the axial direction to form a region 63 g ofsmaller diameter transmitting pressing forces and a lengthening region65 g which serves to improve the fixing of the bush body 51 g in thetool holder, has a larger diameter of the inner lateral surface 59′g, isadjacent to that side of the region 63 g which is remote from the tool,with the formation of a step 67 g, and guides the tool shank during theshrink fitting.

FIG. 15 shows a further variant of a bush body 51 h, the bush wall 55 hof which is likewise closed in an annular manner in the circumferentialdirection. Unlike the variants explained with reference to FIGS. 10 to14, the bush body 51 h has passages 61 h which are situated entirelyinside the bush wall 55 h, extend axially over the entire bush lengthand which, as shown in FIG. 15, may have an elongated shape in crosssection and taper radially inward. Instead of elongated cross-sectionalshapes, however, the passages 61 h may also be circular-cylindricalbores. The outer lateral surface 57 h as well as the inner lateralsurface 59 h are circular-cylindrical, in which case the inner lateralsurface 59 h may again be stepped in the axial direction, as has beenexplained above. The cross-sectional area of the passages 61 h may be sosmall that their thermally insulating properties, on account of thecross-sectional reduction of the heat-transmitting bush cross section,can be disregarded compared with the change in the radially elasticproperties of the bush body 51 h. Just like the bush body in FIGS. 13and 14, the bush body 51 h in FIG. 15 also has properties which improveunshrinking, irrespective of any possible heat-insulating function ofits passages. The bush body 51, in accordance with the bush body 51 g,may thus be made of the material of the tool holder, in particular ofhigh-temperature steel.

In the variants of the bush body which are explained above withreference to FIGS. 10 to 15, the passages or slots, axially on bothsides of the wall region transmitting the pressing forces, end freelyeither in the end face of the bush body or/and on the inner lateralsurface of the bush body. The passages or slots therefore lead past thecircumference of the tool shank sitting in an interference fit in thebush body and can thus be used for feeding lubricant or coolant to thetool during operation of the tool holder. The coolant passages indicatedat 25 in FIG. 1 may be present in the tool holder; however, the coolantfeed may also be effected via the central bore 26 of the tool holder.

FIGS. 16 and 17 show a variant of the tool holder explained withreference to FIGS. 13 and 14, this variant differing from the toolholder in FIGS. 13 and 14 primarily by the fact that the slots 61 istarting from the mounting opening 53 i are incorporated directly in thematerial of the bush part 7. That circumferential region 17 of the bushpart 7 which adjoins the region of the slots 53 i radially on theoutside is again closed in an annular manner. However, its expansionbehavior is now determined by the diameter of the circumferential circleenclosing the radially outer ends of the slots 61 and no longer by thenominal diameter of the mounting opening 53 i. The mounting opening 53 iis thus expanded to a greater degree during heating than in the case ofa bush part without radial slots and can accordingly be produced withgreater tolerances.

The slots 61 i start in an open manner from the tool-side end face 69 iof the bush part 7 and extend axially over at least 5 to 10 mm, butpreferably over the axial length of that region of the bush part 7 whichtransmits interference-fit forces. In the exemplary embodiment in FIGS.16 and 17, the mounting opening 53 i has a uniform diameter over itsentire axial length. However, it may also be stepped, as has beenexplained with reference to FIGS. 13 and 14.

In order to keep the contact area toward the tool shank 11 as large aspossible, this contact area transmitting the torque, the slots 61 i areas narrow as possible, for example between 0.1 and 0.5 mm wide, betterless than 0.4 mm wide, in the circumferential direction. The slot wallsrun parallel to one another in order to configure the web regions 71,situated between the slots 61 i, for a high bending strength duringtorque loading. The number of slots 61 i also constitutes a compromisebetween a sufficiently large contact area of the mounting opening 53 ion the one hand and sufficient bending strength of the web regions 71 onthe other hand. Between 6 and 20 slots 61 i, preferably at least 8 slots61 i, are expediently provided.

The radial depth of the slots 61 i expediently depends on the nominaldiameter of the mounting opening 53 i. Within a range having a nominaldiameter of less than or equal to 3 mm, the radial depth of the slots isselected to be equal to or greater than 0.5 times the nominal diameter.With increasing nominal diameter, the ratio of slot depth to nominaldiameter can be reduced. At a nominal diameter of between 4 mm and 20mm, a radial slot depth of 2 mm is sufficient in the individual case. Ata nominal diameter greater than 25 mm, the slot depth should be at least2.5 mm. The above depth specifications relate to the region of the endface 69 i. Even though the slots 61 i are to have the abovementioneddepth dimensions over the entire axial length of the bush part 7, itnonetheless goes without saying that the radial depth of each slot mayvary in the longitudinal direction. Slots which are adjacent in thecircumferential direction may also have different radial depths.

At least the bush part 7 of the tool holder 1, including the web regions71, is made of high-temperature steel.

1. A tool holder system for holding a rotary tool having an axis of rotation and a fixing shank, the tool holder system comprising: a tool holder having a clamping shank and an annularly closed bush part at a tool-side end of the clamping shank, wherein the bush part comprises a mounting opening coaxial to the axis of rotation for mounting the fixing shank of the rotary tool, a radially outer circumferential region, and a radially inner circumferential region; and induction heating means for heating the radially outer circumferential region of the bush part, by means of eddy currents magnetically induced in the bush part, so as to radially expand the mounting opening to a width enabling the fixing shank to be pushed in or pulled out of the mounting opening, wherein by cooling the bush part, the mounting opening is radially shrinkable to a width holding the fixing shank in an interference fit, and wherein the bush part further comprises a plurality of annularly closed heat-insulating apertures provided in the radially inner circumferential region of the bush part and extending along the axis of rotation for reducing the thermal conductivity of the radially inner circumferential region compared with the radially outer circumferential region, thus reducing the radial heat-conducting cross-section of the bush part, and a plurality of slots extending radially outwardly from the mounting opening each in between a pair of adjacent apertures.
 2. The tool holder system as claimed in claim 1, wherein the apertures contain air or a heat-insulating solid material.
 3. The tool holder system as claimed in claim 1, wherein the radially inner circumferential region, forming the inner circumferential lateral surface of the mounting opening, of the bush part and the radially outer circumferential region are connected to one another in one piece and are made of a homogeneous material.
 4. The tool holder system as claimed in claim 1, wherein cooling means are assigned to the bush part, the cooling means, during heating of the radially outer circumferential region of the bush part, reducing the rate of change of the temperature increase in the radially inner circumferential region of the bush part.
 5. A tool holder system for holding a rotary tool having an axis of rotation and a fixing shank, the tool holder comprising: a tool holder having a clamping shank and an annularly closed bush part at a tool-side end of the clamping shank, wherein the bush part comprises a mounting opening coaxial to the axis of rotation for mounting the fixing shank of the rotary tool, a radially outer circumferential region, and a radially inner circumferential region; and induction heating means for heating the radially outer circumferential region of the bush part, by means of eddy currents magnetically induced in the bush part, so as to radially expand the mounting opening to a width enabling the fixing shank to be pushed in or pulled out of the mounting opening, wherein by cooling the bush part, the mounting opening is radially shrinkable to a width holding the fixing shank in an interference fit, and wherein the bush part further comprises a plurality of heat-insulating apertures provided in the radially inner circumferential region of the bush part and extending along the axis of rotation for reducing the thermal conductivity of a radially inner circumferential region compared with the radially outer circumferential region, thus reducing the radial heat-conducting cross section of the bush part, and a web in circumferential direction in between each pair of adjacent apertures, and wherein the apertures have a radially elongated cross-sectional shape which in circumferential direction is wider adjacent the outer circumferential region than adjacent the inner circumferential region.
 6. The tool holder system as claimed in claim 5, wherein cooling means are assigned to the bush part, the cooling means, during heating of the radially outer circumferential region of the bush part, reducing the rate of change of the temperature increase in the radially inner circumferential region of the bush part.
 7. The tool holder system as claimed in claim 5, wherein the apertures contain air or a heat-insulating solid material.
 8. The tool holder system as claimed in claim 5, wherein the radially inner circumferential region, forming the inner circumferential lateral surface of the mounting opening, of the bush part and the radially outer circumferential region are connected to one another in one piece and are made of a homogeneous material. 